diff --git a/benchmarks/ProjectThemeDiscoveryBench.php b/benchmarks/ProjectThemeDiscoveryBench.php
new file mode 100644
index 0000000..e72d4a7
--- /dev/null
+++ b/benchmarks/ProjectThemeDiscoveryBench.php
@@ -0,0 +1,71 @@
+<?php
+
+declare(strict_types=1);
+
+namespace YiiPress\Benchmarks;
+
+use PhpBench\Attributes\AfterMethods;
+use PhpBench\Attributes\BeforeMethods;
+use PhpBench\Attributes\Iterations;
+use PhpBench\Attributes\Revs;
+use PhpBench\Attributes\Warmup;
+use YiiPress\Build\ProjectThemeDiscovery;
+use YiiPress\Build\Theme;
+use YiiPress\Build\ThemeRegistry;
+
+#[BeforeMethods('setUp')]
+#[AfterMethods('tearDown')]
+final class ProjectThemeDiscoveryBench
+{
+    private string $rootDir;
+    private ProjectThemeDiscovery $discovery;
+
+    public function setUp(): void
+    {
+        $this->rootDir = sys_get_temp_dir() . '/yiipress-project-theme-bench-' . uniqid();
+        mkdir($this->rootDir . '/themes', 0o755, true);
+
+        for ($i = 1; $i <= 50; $i++) {
+            mkdir($this->rootDir . '/themes/theme-' . $i);
+        }
+
+        $this->discovery = new ProjectThemeDiscovery();
+    }
+
+    public function tearDown(): void
+    {
+        $this->removeDir($this->rootDir);
+    }
+
+    #[Revs(100)]
+    #[Iterations(3)]
+    #[Warmup(1)]
+    public function benchRegisterProjectThemes(): void
+    {
+        $registry = new ThemeRegistry();
+        $registry->register(new Theme('minimal', '/built-in/minimal'));
+
+        $this->discovery->register($registry, $this->rootDir . '/themes');
+    }
+
+    private function removeDir(string $path): void
+    {
+        if (!is_dir($path)) {
+            return;
+        }
+
+        $iterator = new \RecursiveIteratorIterator(
+            new \RecursiveDirectoryIterator($path, \FilesystemIterator::SKIP_DOTS),
+            \RecursiveIteratorIterator::CHILD_FIRST,
+        );
+        foreach ($iterator as $item) {
+            if ($item->isDir()) {
+                rmdir($item->getPathname());
+            } else {
+                unlink($item->getPathname());
+            }
+        }
+
+        rmdir($path);
+    }
+}
diff --git a/docs/engine.md b/docs/engine.md
index 76626ee..91610fb 100644
--- a/docs/engine.md
+++ b/docs/engine.md
@@ -193,7 +193,7 @@ The source-open overlay resolves the browser path through the build manifest, ve
 
 ## Theme Registration
 
-Project-local templates under `content/templates/` are registered automatically as the `local` theme. Engine-level or distributable themes are registered in [Yii3 DI](https://yiisoft.github.io/docs/guide/concept/di-container.html):
+Project themes under `<project>/themes/<name>/` are registered automatically by directory name. Project-local templates under `content/templates/` are registered automatically as the `local` theme. Engine-level themes may still be registered in [Yii3 DI](https://yiisoft.github.io/docs/guide/concept/di-container.html):
 
 ```php
 use YiiPress\Build\Theme;
@@ -204,11 +204,10 @@ return [
     ThemeRegistry::class => DynamicReference::to(static function (): ThemeRegistry {
         $registry = new ThemeRegistry();
         $registry->register(new Theme('minimal', dirname(__DIR__, 3) . '/themes/minimal'));
-        $registry->register(new Theme('fancy', '/path/to/fancy-theme'));
 
         return $registry;
     }),
 ];
 ```
 
-Template resolution checks the active theme first, then falls back through registered themes. This lets a project override one template while keeping the rest of the bundled theme.
+For binary users, install a reusable theme as `themes/fancy/` and set `theme: fancy` in `content/config.yaml`. Template resolution checks the active theme first, then falls back through registered themes. This lets a project override one template while keeping the rest of the bundled theme.
diff --git a/docs/templates.md b/docs/templates.md
index 6f3c8ea..1ca585b 100644
--- a/docs/templates.md
+++ b/docs/templates.md
@@ -43,7 +43,7 @@ Use `$h()` for text that should be escaped. Rendered Markdown content in `$conte
 
 ## Themes
 
-A theme is a named set of template files. YiiPress ships with the built-in `minimal` theme. A project-local `content/templates/` directory is automatically available as the `local` theme.
+A theme is a named set of template files. YiiPress ships with the built-in `minimal` theme. Project themes under `themes/<name>/` are registered automatically, and a project-local `content/templates/` directory is automatically available as the `local` theme.
 
 ### Theme resolution order
 
@@ -53,7 +53,28 @@ When YiiPress renders a page, it chooses templates in this order:
 2. **Site-level default theme** — set via `theme` in `config.yaml`.
 3. **Built-in `minimal` theme** — fallback when a template is missing.
 
-Within a theme, YiiPress uses the requested file when it exists and falls back to other registered themes when it does not. That means a local theme can override only `entry.php` and keep every other page type from `minimal`.
+Within a theme, YiiPress uses the requested file when it exists and falls back to other registered themes when it does not. That means a project theme or local theme can override only `entry.php` and keep every other page type from `minimal`.
+
+### Project themes
+
+Install reusable themes into the project root:
+
+```
+themes/
+└── docs/
+    ├── entry.php
+    ├── partials/
+    ├── assets/
+    └── translation/
+```
+
+Use the directory name as the theme name:
+
+```yaml
+theme: docs
+```
+
+Theme directory names may contain letters, numbers, `_`, and `-`, and must start with a letter or number. If a project theme has the same name as an already registered built-in theme, the built-in theme is kept.
 
 ### Local theme
 
diff --git a/roadmap.md b/roadmap.md
index f36c641..05991e4 100644
--- a/roadmap.md
+++ b/roadmap.md
@@ -64,6 +64,7 @@
 ## Priority 4: Templates and theming
 
 - [x] Theme system — installable/distributable themes
+- [x] Auto-register project themes from `themes/<name>/` for binary users
 - [x] Template partials/includes support
 - [x] Template helper functions documentation
 - [x] Multiple layout support (per-entry layout selection via front matter)
diff --git a/src/Build/ProjectThemeDiscovery.php b/src/Build/ProjectThemeDiscovery.php
new file mode 100644
index 0000000..395a212
--- /dev/null
+++ b/src/Build/ProjectThemeDiscovery.php
@@ -0,0 +1,66 @@
+<?php
+
+declare(strict_types=1);
+
+namespace YiiPress\Build;
+
+use FilesystemIterator;
+use SplFileInfo;
+
+use function array_values;
+use function is_dir;
+use function ksort;
+use function preg_match;
+
+final readonly class ProjectThemeDiscovery
+{
+    /**
+     * @return list<Theme>
+     */
+    public function discover(string $themesDir): array
+    {
+        if (!is_dir($themesDir)) {
+            return [];
+        }
+
+        $themes = [];
+        $iterator = new FilesystemIterator($themesDir, FilesystemIterator::SKIP_DOTS);
+        foreach ($iterator as $item) {
+            /** @var SplFileInfo $item */
+            if (!$item->isDir()) {
+                continue;
+            }
+
+            $name = $item->getFilename();
+            if (!$this->isValidThemeName($name)) {
+                continue;
+            }
+
+            $themes[$name] = new Theme($name, $item->getPathname());
+        }
+
+        ksort($themes);
+
+        return array_values($themes);
+    }
+
+    public function register(ThemeRegistry $registry, string $themesDir): int
+    {
+        $registered = 0;
+        foreach ($this->discover($themesDir) as $theme) {
+            if ($registry->has($theme->name)) {
+                continue;
+            }
+
+            $registry->register($theme);
+            $registered++;
+        }
+
+        return $registered;
+    }
+
+    private function isValidThemeName(string $name): bool
+    {
+        return preg_match('/^[A-Za-z0-9][A-Za-z0-9_-]*$/D', $name) === 1;
+    }
+}
diff --git a/src/Console/BuildCommand.php b/src/Console/BuildCommand.php
index 4e1e959..f6fed48 100644
--- a/src/Console/BuildCommand.php
+++ b/src/Console/BuildCommand.php
@@ -15,6 +15,7 @@
 use YiiPress\Build\DateArchiveWriter;
 use YiiPress\Build\NotFoundPageWriter;
 use YiiPress\Build\NavigationPager;
+use YiiPress\Build\ProjectThemeDiscovery;
 use YiiPress\Build\RedirectPageWriter;
 use YiiPress\Build\RobotsTxtGenerator;
 use YiiPress\Build\SearchIndexGenerator;
@@ -219,6 +220,8 @@ protected function execute(InputInterface $input, OutputInterface $output): int
             return ExitCode::DATAERR;
         }
 
+        (new ProjectThemeDiscovery())->register($this->themeRegistry, $rootPath . '/themes');
+
         $localTemplatesDir = $contentDir . '/templates';
         if (is_dir($localTemplatesDir)) {
             $this->themeRegistry->register(new Theme('local', $localTemplatesDir));
diff --git a/tests/Unit/Build/ProjectThemeDiscoveryTest.php b/tests/Unit/Build/ProjectThemeDiscoveryTest.php
new file mode 100644
index 0000000..49065a8
--- /dev/null
+++ b/tests/Unit/Build/ProjectThemeDiscoveryTest.php
@@ -0,0 +1,100 @@
+<?php
+
+declare(strict_types=1);
+
+namespace YiiPress\Tests\Unit\Build;
+
+use FilesystemIterator;
+use PHPUnit\Framework\TestCase;
+use RecursiveDirectoryIterator;
+use RecursiveIteratorIterator;
+use SplFileInfo;
+use YiiPress\Build\ProjectThemeDiscovery;
+use YiiPress\Build\Theme;
+use YiiPress\Build\ThemeRegistry;
+
+use function PHPUnit\Framework\assertCount;
+use function PHPUnit\Framework\assertSame;
+
+final class ProjectThemeDiscoveryTest extends TestCase
+{
+    private string $rootDir;
+
+    protected function setUp(): void
+    {
+        $this->rootDir = sys_get_temp_dir() . '/yiipress-project-themes-test-' . uniqid();
+        mkdir($this->rootDir . '/themes', 0o755, true);
+    }
+
+    protected function tearDown(): void
+    {
+        $this->removeDir($this->rootDir);
+    }
+
+    public function testDiscoversProjectThemesInDeterministicOrder(): void
+    {
+        mkdir($this->rootDir . '/themes/zeta');
+        mkdir($this->rootDir . '/themes/alpha');
+        mkdir($this->rootDir . '/themes/docs_theme');
+        file_put_contents($this->rootDir . '/themes/readme.md', 'not a theme');
+
+        $themes = (new ProjectThemeDiscovery())->discover($this->rootDir . '/themes');
+
+        assertSame(['alpha', 'docs_theme', 'zeta'], array_map(static fn (Theme $theme): string => $theme->name, $themes));
+    }
+
+    public function testSkipsInvalidThemeNames(): void
+    {
+        mkdir($this->rootDir . '/themes/good-theme');
+        mkdir($this->rootDir . '/themes/.hidden');
+        mkdir($this->rootDir . '/themes/bad.name');
+
+        $themes = (new ProjectThemeDiscovery())->discover($this->rootDir . '/themes');
+
+        assertCount(1, $themes);
+        assertSame('good-theme', $themes[0]->name);
+    }
+
+    public function testReturnsEmptyListWhenThemesDirectoryIsMissing(): void
+    {
+        $themes = (new ProjectThemeDiscovery())->discover($this->rootDir . '/missing');
+
+        assertSame([], $themes);
+    }
+
+    public function testRegistersThemesWithoutOverwritingExistingNames(): void
+    {
+        mkdir($this->rootDir . '/themes/minimal');
+        mkdir($this->rootDir . '/themes/fancy');
+        $registry = new ThemeRegistry();
+        $registry->register(new Theme('minimal', '/built-in/minimal'));
+
+        $registered = (new ProjectThemeDiscovery())->register($registry, $this->rootDir . '/themes');
+
+        assertSame(1, $registered);
+        assertSame('/built-in/minimal', $registry->get('minimal')->path);
+        assertSame($this->rootDir . '/themes/fancy', $registry->get('fancy')->path);
+    }
+
+    private function removeDir(string $path): void
+    {
+        if (!is_dir($path)) {
+            return;
+        }
+
+        $iterator = new RecursiveIteratorIterator(
+            new RecursiveDirectoryIterator($path, FilesystemIterator::SKIP_DOTS),
+            RecursiveIteratorIterator::CHILD_FIRST,
+        );
+        foreach ($iterator as $item) {
+            /** @var SplFileInfo $item */
+            if ($item->isDir()) {
+                rmdir($item->getPathname());
+            } else {
+                unlink($item->getPathname());
+            }
+        }
+
+        rmdir($path);
+    }
+}
diff --git a/tests/Unit/Console/BuildCommandTest.php b/tests/Unit/Console/BuildCommandTest.php
index 6670f79..5715866 100644
--- a/tests/Unit/Console/BuildCommandTest.php
+++ b/tests/Unit/Console/BuildCommandTest.php
@@ -143,6 +143,55 @@ public function testBuildHooksAreDispatched(): void
         assertFileExists($outputDir . '/index/index.html');
     }
 
+    public function testBuildAutoRegistersProjectThemes(): void
+    {
+        $tempDir = sys_get_temp_dir() . '/yiipress-build-project-theme-test-' . uniqid();
+        $contentDir = $tempDir . '/content';
+        $outputDir = $tempDir . '/output';
+        $themeDir = $tempDir . '/themes/docs';
+        mkdir($contentDir, 0o755, true);
+        mkdir($themeDir, 0o755, true);
+        $this->tempContentDirs[] = $tempDir;
+
+        file_put_contents($contentDir . '/config.yaml', "title: Themed Site\nbase_url: https://example.com\nlanguages: [en]\ntheme: docs\n");
+        file_put_contents($contentDir . '/index.md', "---\ntitle: Home\npermalink: /\n---\n\nProject theme content.\n");
+        file_put_contents(
+            $themeDir . '/entry.php',
+            <<<'PHP'
+<?php
+declare(strict_types=1);
+?>
+<html><body class="docs-theme"><?= $content ?></body></html>
+PHP,
+        );
+
+        $themeRegistry = new ThemeRegistry();
+        $themeRegistry->register(new Theme('minimal', dirname(__DIR__, 3) . '/themes/minimal'));
+        $templateResolver = new TemplateResolver($themeRegistry);
+        $pipeline = new ContentProcessorPipeline(new MarkdownProcessor(new MarkdownRenderer()));
+        $command = new BuildCommand(
+            rootPath: $tempDir,
+            contentPipeline: $pipeline,
+            feedPipeline: new ContentProcessorPipeline(new MarkdownProcessor(new MarkdownRenderer())),
+            themeRegistry: $themeRegistry,
+            templateResolver: $templateResolver,
+        );
+        $tester = new CommandTester($command);
+
+        $exitCode = $tester->execute([
+            '--content-dir' => $contentDir,
+            '--output-dir' => $outputDir,
+            '--workers' => '1',
+            '--no-cache' => true,
+        ]);
+
+        assertSame(0, $exitCode, $tester->getDisplay());
+        $html = file_get_contents($outputDir . '/index.html');
+        assertNotFalse($html);
+        assertStringContainsString('class="docs-theme"', $html);
+        assertStringContainsString('Project theme content.', $html);
+    }
+
     public function testBuildReportsInvalidSiteConfigWithoutTrace(): void
     {
         $tempDir = sys_get_temp_dir() . '/yiipress-build-invalid-config-test-' . uniqid();